1. Field of the Invention
This invention relates to compositions useful in the making of plastic lenses having high thermal resistance, high impact resistance, low water absorption and good moldability.
2. Description of the Prior Art
Because of their unique properties such as easy moldability and light weight, plastic lenses have come to be widely used in optical products. In particular, it is desirable that eyeglasses are light in weight. In recent years, therefore, a resin comprising diethylene glycol bisallyl carbonate (commercially available under the tradename of CR-39 from PPG Co., Ltd.) is being used as the chief material for the making of plastic eyeglasses.
However, high refractive indices and high productivity have lately come to be required of plastic lenses. As a result, plastic lenses made by using various monomers and oligomers in place of CR-39 have been proposed.
Important properties required of plastic lenses include high thermal resistance, high impact resistance, low water absorption, molding surface accuracy, dyeability and the like. Conventionally, monomers and oligomers capable of imparting highly elastic structures such as the ether, urethane, ester and carbonate linkages have been used as components for improving impact resistance and dyeability. Among others, the use of low-viscosity di(meth)acrylates having an ether linkage in the molecule and capable of improving casting workability has been proposed (Japanese Patent Laid-Open No. 16813/'89).
Typical examples of such di(meth)acrylate monomers capable of producing a polyether structure are polyethylene glycol di(meth)acrylates and polypropylene glycol di(meth)acrylates. When these monomers are used, the impact resistance and dyeability of the resulting lenses are improved in proportion to the increasing number of repeating ethylene oxide or propylene oxide units.
On the other hand, however, this approach has posed a problem concerning the retention of high thermal resistance, low water absorption and high surface accuracy which are required of lenses. In order to achieve high thermal resistance and low water absorption, it generally suffices to impart low water absorption properties to the polymer. Improvement of the low water absorption properties of a polymer has been achieved by introducing a hydrocarbon chain, aromatic ring or halogen atom into the molecule thereof (Japanese Patent Laid-Open No. 66401/'82). This method brings about high thermal resistance and low water absorption, but causes a reduction in impact resistance and dyeability.
In order to solve the above-described problems, the present inventors have made intensive studies and have found that a certain polybutylene glycol dimethacrylate is a monomer capable of producing a polymer having a well-balanced combination of impact resistance and water absorption, and a certain composition containing this monomer can yield a product having high impact resistance and low water absorption. Moreover, it has also been found that, by applying this composition to the making of plastic lenses, there can be obtained plastic lenses having high impact resistance, good dyeability, high thermal resistance and low water absorption. However, this composition fails to achieve satisfactorily high surface accuracy (i.e., coincidence between the lens curvature and the design curvature) which is a property required of lenses.
The present inventors continued to make intensive studies and have found a composition containing an urethane poly(meth)acrylate or an epoxy poly(meth)acrylate for improving thermal resistance, a polybutylene glycol di(meth)acrylate for improving impact resistance, low water absorption and dyeability, and a mono(meth)acrylate of alicyclic hydrocarbon monoalcohol or aromatic monoalcohol for improving surface accuracy which can yield excellent lens products. Accordingly, the present inventors completed the present invention.